-LRB- CNN -RRB- -- Thousands of mourners and supporters crowded hillsides near Srebrenica , Bosnia-Herzegovina , Sunday , to mark the 15th anniversary of the massacre of nearly 8,000 men and boys .

More than 50,000 people witnessed the burial of 775 newly identified genocide victims at the Centre Potocari -- the official Srebrenica Genocide Memorial where 5,000 genocide victims are buried -- a few miles outside the city , according to Bosnia-Herzegovina 's official news agency , Fena .

In 1995 , Potocari served as a safe-haven for Bosnian Muslims . They were under the protection of a U.N.-Dutch-led mission .

The massacre of men and boys occurred when ethnic Serb troops overran the United Nations safe area .

The five-day slaughter was the worst European massacre since World War II and was described by the U.N. War Crimes Tribunal as `` the triumph of evil . '' Witnesses , human rights investigators and court testimonies documented the summary executions , as well the rape and murder of women .

Sunday 's burial was attended by top international officials , including a U.S. delegation whose members read a statement from President Barack Obama . In the statement , Obama called the Srebrenica massacre an `` unimaginable '' tragedy that has left an indelible `` stain on our collective conscience . ''

`` Fifteen years ago today , despite decades of pledges of ` never again , ' 8,000 men and boys were murdered in these fields and hills , '' said in the statement .

`` They were brothers , sons , husbands , and fathers , and they all became victims of genocide . ''

Obama also called on all governments to `` redouble their efforts '' to find and prosecute those responsible for the tragedy .

`` This includes Ratko Mladic , who presided over the killings and remains at large , '' he said , referring to the former Bosnian Serb army commander .

Mladic has been charged with genocide and remains a fugitive . Bosnian Serb president Radovan Karadzic is now on trial at the tribunal in The Hague .

Fena news agency reported that Boris Tadic , president of Serbia , also attended the burial memorial of the genocide victims and promised to find Serb war criminals .

`` I , as the president of Serbia , will never give up finding them , and especially Ratko Mladic . When that ends , I will consider a part of my job completed , '' Tadic said .

`` But before that , it is important that people reach out to one another and to continue living as people . A lot of time has passed , and I have done everything possible for Mladic to be arrested , '' he said .

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Obama : Massacre leaves `` stain on our collective conscience ''

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Thousands watch burial of 775 recently identified genocide victims

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Ratko Mladic has been charged with genocide and remains a fugitive

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Bosnian Serb president Radovan Karadzic is now on trial at the tribunal in The Hague .